Photolithography is a process used in semiconductor integrated circuit (IC) device fabrication to produce device structures on semiconductor or other substrates. Distortions of device structures are becoming evident in view of the shrinking of the dimensions of the device structures as compared to the radiation wavelengths used during photolithography. The challenges presented by sub-wavelength photolithography include image distortion in the form of line-end shortenings, corner rounding, isolated/dense proximity effects, and adverse impacts on the depth of focus (DOF). One source of distortion is due to light scattered or otherwise affected by adjacent structures. Distortion in size and shape of the projected image exhibited by this phenomenon is called proximity effect.
In optical proximity correction (OPC), a resolution enhancement technique (RET) using scattering bars, has been introduced to counter proximity effects and to reduce distortion. Scattering bars are sub-resolution assist features (SRAF) that are placed on a mask (also referred to as a reticle or photo-mask) adjacent to isolated features and/or semi-isolated features. Isolated and semi-isolated design features, such as metal lines, trenches, or gate polysilicon, are generally exposed and/or printed on the substrate at a feature size significantly different from the same design feature surround by other nearby features. This phenomenon is known as an isolated/dense proximity effect. The use of scattering bars enables these isolated and/or semi-isolated design features to form similar to how dense features are formed. In this manner, the usable resolution of an imaging system may be extended without decreasing the radiation wavelength or increasing a numerical aperture of the imaging tool, although such processes can be used for additional benefit.
One type of conventional scattering bars is to have narrow lines placed in parallel with and adjacent to edges (or sides) of an isolated design feature. These types of scattering bars are commonly called edge scattering bars (ESB). Edge scattering bars are sometimes used in conjunction with geometry patterns (GP) which are small holes or islands on mask placed oblique to an isolated feature. ESBs and GPs are typically arranged within an area closely surrounding an isolated feature. As such, these assist features are very small in sizes and present challenges for mask fabrication.